The Transforming Growth Factor-beta (TGFβ) superfamily, whose members include TGFβs, activins and bone morphogenetic proteins (BMPs), have wide ranging effects on cells of diverse origins (Attisano and Wrana, 1998; Heldin et al., 1997; Kretzschmar and Massagué, 1998). Signaling by these secreted factors is initiated upon interaction with a family of cell-surface transmembrane serine/threonine kinases, known as type I and type II receptors. Ligand induces formation of a typeI/typeII heteromeric complex which permits the constitutively active type II receptor to phosphorylate, and thereby activate, the type I receptor (Wrana et al., 1994). This activated type I receptor then propagates the signal to a family of intracellular signaling mediators known as Smads (Attisano and Wrana, 1998; Heldin et al., 1997; Kretzschmar and Massagué, 1998).
The first members of the Smad family identified in invertebrates were the Drosophila MAD and the C. elegans sma genes (sma-2, sma-3 and sma4; Savage et al., 1996; Sekelsky et al., 1995). Currently, the family includes additional invertebrate Smads, as well as nine vertebrate members, Smadi through 9 (Attisano and Wrana, 1998; Heldin et al., 1997; Kretzschmar and Massagué, 1998). Smad proteins contain two conserved amino (MH1) and carboxy (MH2) terminal regions separated by a more divergent linker region. In general, Smad proteins can be subdivided into three groups; the receptor-regulated Smads, which include Smad 1, 2, 3, 5 and 8, Mad, sma-2 and sma-3; the common Smads, Smad4 and Medea, and the antagonistic Smads, which include Smad6, 7 and 9, DAD and daf-3 (Heldin et al., 1997; Nakayama et al., 1998; Patterson et al., 1997).
Numerous studies with vertebrate Smad proteins have provided insights into the differential functions of these proteins in mediating signaling. Receptor-regulated Smads are direct substrates of specific type I receptors and the proteins are phosphorylated on the last two serines at the carboxy-terminus within a highly conserved SSXS motif (Abdollah et al., 1997; Kretzschmar et al., 1997; Liu et al., 1997b; Macias-Silva et al., 1996; Souchelnytskyi et al., 1997). Interestingly, Smad2 and Smad3 are substrates of TGFβ or activin receptors and mediate signaling by these ligands (Liu et al., 1997b; Macias-Silva et al., 1996; Nakao et al., 1997a), whereas Smad1, 5 and 8 appear to be targets of BMP receptors and thereby propagate BMP signals (Chen et al., 1997b; Hoodless et al., 1995; Kretzschmar et al., 1997; Nishimura et al., 1998). Once phosphorylated, these Smads bind to the common Smad, Smad4, which lacks the carboxy-terminal phosphorylation site and is not a target for receptor phosphorylation (Lagna et al., 1996; Zhang et al., 1997). Heteromeric complexes of the receptor-regulated Smad and Smad4 translocate to the nucleus where they function to regulate the transcriptional activation of specific target genes. The antagonist Smads, Smad6, 7 and 9 appear to function by blocking ligand-dependent signaling by preventing access of receptor-regulated Smads to the type I receptor or possibly by blocking formation of heteromeric complexes with Smad4 (reviewed in Heldin et al., 1997).
Analysis of the nuclear function of Smads has demonstrated that Smads can act as transcriptional activators and that some Smads, including Drosophila Mad, and the vertebrate Smad3 and Smad4, can bind directly to DNA, albeit at relatively low specificity and affinity (Dennler et al., 1998; Kim et al., 1997; Labbé et al., 1998; Yingling et al., 1997; Zawel et al., 1998).
Localization of Smads is critical in controlling their activity and Smad phosphorylation by the type I receptor regulates Smad activity by inducing nuclear accumulation (Attisano and Wrana, 1998; Heldin et al., 1997; Kretzschmar and Massagué, 1998). However, little is known about how Smad localization is controlled prior to phosphorylation and how this might function in modulating receptor interactions with its Smad substrates.